VFD Versus LCD: Where Each Display Type Still Applies

Vacuum fluorescent displays have been declared obsolete for twenty years and remain in production, because they do one thing extremely well: they glow. A VFD is bright, self-emitting and readable from a very wide angle, with a look that no backlit LCD reproduces exactly. Where that look is the product and where the information is fixed, the VFD survives, and understanding why makes the choice clearer in both directions.

Part of the Optical, Backlight & Panel Technology guide — 30 articles on bonding, cover glass, brightness and panel choice.

What is a VFD display?

A vacuum tube that forms characters from phosphor.

A vacuum fluorescent display is a small evacuated tube containing filaments, grids and anodes coated with a phosphor. Electrons from the heated filament are accelerated and land on the phosphor, which emits light. Because the light is generated at the character itself, the display is self-emitting like an OLED and needs no backlight or light guide, and because the emitting layer is thin and on the front surface, the viewing angle is very wide. The display families around it are surveyed in the display and touch solutions centre.

How a VFD makes light

Thermionic emission, then phosphorescence.

A low-voltage filament heats to emission temperature, the grids control which anode segments are addressed, and the phosphor converts the electron energy into visible light. The practical consequences follow from the physics: the display needs a filament supply, the drive voltages are higher than an LCD’s, and the brightness is uniform across the glass because each segment generates its own light. It also means the display is a glass tube, with the mechanical and sealing considerations that implies at the edge of a panel.

Where VFD still wins: brightness and angle

Emissive light does not have to compete with reflections.

An LCD has to modulate light from behind it while rejecting the ambient reflections in front, so contrast falls as the room gets brighter. An emissive display adds light to the environment it is competing with, so it holds its contrast far longer, and it does so over a viewing cone that a twisted-nematic LCD cannot match. For a dashboard, a hi-fi front panel, a vending machine or a piece of test equipment read from an angle in a bright room, the VFD still has no equal at the same cost. The measurement discipline behind the comparison is the same as for any display and is described in measuring readability in real ambient light.

Where LCD wins: cost, formats and power

Almost everywhere the information is variable.

An LCD can be made in any size, any resolution and any aspect ratio, with colour filters and a full colour palette, at a cost per area that VFD cannot approach. It runs from a single low voltage, needs no filament and draws a fraction of the current for the same information. Where the display must show graphics, images, a variable menu or a touch interface, the question does not arise. The relative position of the remaining panel families is set out in which is best for HMI, and the emissive alternative in OLED versus LCD for industrial HMI.

Colour, fonts and the VFD look

The aesthetic is part of the specification.

VFD phosphors give the characteristic blue-green, and the segment shapes are designed rather than rendered, so a seven-segment VFD has a particular form that a dot-matrix LCD cannot reproduce without visible pixels. Where a product’s identity depends on that look, an LCD substitute is a visible downgrade even if it is technically superior, and the answer is usually to keep a VFD or to accept the change as a deliberate restyle. Font choice on an LCD equivalent should follow the same principle as the display and touch solutions centre guidance for character and graphic modules.

Lifetime and the filament

The cathode ages, and the brightness follows.

A VFD’s life is limited by the filament and by the phosphor, both of which decline with use until brightness falls below a usable figure. Unlike an LCD, whose backlight can be replaced in principle, a VFD is a sealed tube and the decline is not reversible. The life is long enough for many applications and is stated as a brightness half-life, so the specification should quote the figure and the drive condition that produces it, rather than an unqualified hour count. Where the product has to run for a very long time, that comparison with LCD options belongs in the design review.

Temperature, voltage and drive electronics

Higher voltages and a filament supply.

A VFD needs a filament supply and a higher anode voltage than an LCD, which adds a transformer or a switching supply and its own compliance considerations. Cold temperature reduces emission until the filament warms, so the display can be dim for a moment after power-up in a cold environment. None of that is difficult, but it changes the power architecture and it should be designed in rather than discovered. The safety framework for a mains-powered appliance containing such a supply is covered by IEC 62368-1, with EMC obligations under the EU EMC Directive.

Are VFDs still manufactured?

Yes, but the supply is narrow.

VFD production continues for the specific applications that value the look, especially in the instrument, appliance and automotive aftermarket, but the number of suppliers and the variety of standard formats have both shrunk. A design that commits to a VFD without confirming availability for the product’s life is taking a supply risk that may be larger than the aesthetic gain. Where the display is a standard module, the availability and tracking information published by distributors and market researchers such as TrendForce is worth checking, and the documentation expectations of bodies such as the CIE and NIST apply to any optical specification that accompanies the change.

Replacing a VFD with an LCD

Four things have to be reconciled.

• The optical first: an LCD is not self-emitting, so brightness and angle must be re-specified with the new backlight.
• The mechanical envelope: an LCD module with a backlight is usually thicker than the VFD it replaces.
• The electrical interface: the drive is completely different, so the controller and firmware change too.
• The look: segment shapes and colour are product features, so a faithful replacement may need a custom LCD segment design.

Where the replacement must preserve an existing control board, the practical route is a drop-in LCD module with a compatible interface; where it may change, a graphic panel gives more freedom. In either case the acceptance is measured with the panel in the product, since the housing and the front glass change the result, which is the same principle as any workmanship question governed by IPC practice.

Selection rules

Three questions decide it.

First, does the information change in any way other than its digits? If yes, choose an LCD. Second, is the product’s look part of its value, and is the ambient light high with a wide viewing angle? If yes, a VFD still earns its place. Third, is the product’s life longer than the VFD supply can be assured? If yes, the replacement should be planned now rather than at the next production run. The same questions, applied to the emissive-versus-transmissive trade, are the subject of active versus passive LCD displays and of the reflective alternative in e-paper and reflective displays.